Viral development, colicine production, prokaryotic cell division, and oncogenesis are induced by the same class of agents, i.e., radiation, mitomycin, thymineless death, and chemicals affecting DNA synthesis and repair, and may operate through common molecular mechanisms. The writer proposed a mechanism of lambda virus induction which may have widespread applicability to other systems mentioned above. Lambda repressor is assumed to have affinity not only for lambda operators, but also for single strand lesions in the host DNA. Induction, i.e., derepression, would occur when the capacity of the cell to repair the lesions is exceeded. Significant evidence supporting this model was provided by in vitro binding studies in which purified DNA from induced E. coli competed with lambda DNA for binding with lambda repressor. We have established that repressor has high affinity and binds to single strand gaps in duplex DNA. In contrast, the non-inducible mutant repressor ( lambda ind) has 10 times less affinity for this lesion. We propose: 1) To continue the study of in vitro binding of wild type and mutant repressors to gapped DNA, employing diverse methods, i.e., column chromatography, fluorescence spectrometry, antibody interactions, ets.. Questions to be answered are: Which domain interacts with non-operator DNA?; Does repressor undergo an allosteric change when bound to gapped DNA?; Is there competition for gaps between repressor, recA protein and and E. coli binding protein? 2) To study the participation of gapped DNA and recA protein in the proteolytic cleavage of the repressor, in reaction mixtures with purified components. 3) To isolate mutants displaying specific defects with respect to proteolytic cleavage and analyze them. 4) To provide conditions in the cell that may establish which of the following products or reactions are essential to induction: a) DNA degradation products; b) single strand gaps; c) proteolysis. 5) To extend this project to the isolation of other radiation-inducible repressors.